Anisotropic polymer films comprising a polymerized liquid crystal material with uniform orientation are known in prior art. They are usually prepared by coating a thin layer of a polymerizable liquid crystal mixture onto a substrate, aligning the mixture into uniform orientation and polymerizing the mixture.
For specific applications it is required to induce planar alignment in the liquid crystal layer, i.e., where the liquid crystal molecules are oriented substantially parallel to the layer. The alignment is then frozen in by polymerizing the liquid crystal mixture in situ. For example, oriented films or layers of polymerized nematic liquid crystal material with planar alignment are useful as A-plate compensators or polarizers. Another important application is oriented films or layers of polymerized cholesteric liquid crystal material having twisted molecular structure. If the cholesteric material has planar alignment, these films show selective reflection of light where the reflection colour is dependent on the viewing angle. They can be used, for example, as circular polarizers, colour filters or for the preparation of effect pigments for decorative or security applications. Furthermore, patterned films are known comprising regions of different orientation direction. These can be used in optical elements as mentioned above for decorative purposes or in security devices.
Planar alignment can be achieved, for example, by treatment of the substrate onto which.,the liquid crystal material is coated. The most usual method of surface treatment is to rub the substrate surface prior to application of the liquid crystal material. In case of rod-shaped liquid crystal molecules, these will align themselves with their long axes parallel to the rubbing direction. Alternatively, it is possible to apply an alignment layer, for example of polyimide, to the substrate, which can then subsequently be rubbed or which will induce the desired alignment. Other methods are the application of shear forces or the addition of surface active compounds to the liquid crystal material.
Reviews of conventional alignment techniques are given, for example, by I. Sage in “Thermotropic Liquid Crystals”, edited by G. W. Gray, John Wiley & Sons, 1987, pages 75–77, and by T. Uchida and H. Seki in “Liquid Crystals—Applications and Uses Vol. 3”, edited by B. Bahadur, World Scientific Publishing, Singapore 1992, pages 1–63. A review of alignment materials and techniques is given by J. Cognard, Mol. Cryst. Liq. Cryst. 78, Supplement 1 (1981), pages 1–77.
However, the methods of the prior art have several drawbacks. The use of alignment layers or additives in the liquid crystal material leads to increased costs. Rubbing of the substrate or the application of shear forces are usually carried out in one direction, so that the liquid crystal molecules will adopt planar alignment into one preferred direction over the entire film. The formation of patterned films comprising regions of different orientation is difficult with these methods.
Other methods to prepare patterned films known in prior art imply photoorientation or photoisomerisation of the liquid crystal material. However, this requires the additional use of photoisomerisable or photoorientating materials and of special techniques like photomasking or photopolymerisation under linearly polarised light.
Rubbing of a substrate also implies several drawbacks when used in mass production of liquid crystal polymer films. Therein, usually a flexible long film substrate is continuously unwound from a roll and rubbed directly before being coated with a polymerizable liquid crystal material, which is then polymerized and may be removed from the substrate. Whereas rubbing and thus alignment of the liquid crystal molecules can easily be achieved by conveying the substrate over a rubbing roller or between two rollers, alignment at any desired angle to the substrate moving direction requires more complicated rubbing stages, where the rollers can be positioned at an angle to the conveying direction of the substrate. Also, rubbing in a direction at right angles to the substrate conveying direction is not possible with this method.
An aim of the present invention is to provide a method of uniform alignment of liquid crystal material in the production of polymer films, wherein this method does not have the drawbacks of the prior art, allows alignment in any desired direction within the film plane and also the formation of patterned films, and is suitable for mass production and coating on a continuously conveying substrate. Other aims are directly evident to the expert from the following description.
The inventors have found that these aims can be achieved and the above drawbacks can be overcome by using a substrate with a structured surface, for example with a surface profile in the form of gratings or fine grooves. A polymerizable liquid crystal material coated onto the substrate will spontaneously align in the direction of the grating, and the alignment can then be fixed by polymerization. The surface profiles or gratings can be formed by known techniques, like for example embossing, photolithography, or interferography. The direction of the gratings can be freely chosen at any desired angle, and patterned structures may also be formed.
The use of substrates with a profiled or structured surface has been described in prior art for the alignment of low molar mass liquid crystals in switchable or thermochromic display devices. For example, U.S. Pat. No. 4,834,500 discloses a method of aligning a thermochromic cholesteric liquid crystal material between flexible walls having a surface that is profiled with a series of fine grooves and ridges, whereby the molecules of the cholesteric material are aligned substantially parallel to the wall. U.S. Pat. No. 5,724,113 describes a method to induce tilted alignment in a nematic, smectic or cholesteric liquid crystal cell by providing an alignment layer with an asymmetric surface grating onto the cell walls. U.S. Pat. No. 5,754,264 discloses a method of surface treatment to achieve a pretilt in a ferroelectric liquid crystal cell by providing symmetrical or asymmetrical monogratings to the surface of the cell walls. U.S. Pat. No. 5,764,325 discloses a method to achieve surface alignment and surface tilt in a twisted nematic liquid crystal cell by providing a grating of grooves with an asymmetric profile to the surface of the cell walls. WO 97/14990 and WO 99/34251 describe a bistable liquid crystal cell wherein the cell wall is provided with a surface alignment grating that permits the liquid crystal molecules to adopt two different pretilt angles.
However, these documents do not mention polymerizable liquid crystal materials or the formation of polymer films with uniform or patterned orientation in large scale production.